The purpose of this research is to describe the role of certain reflex systems in the control of unrestrained, functional human behaviors. The specific behavior to be examined is stepping-down - an act necessary for full mobility. Healthy adult volunteers will participate in the experiment which will be organized around the biomechanical and electrophysiological parameters of interest. These are a) force of the impact on the lower level, b) downward velocity of the body, c) angle at which the ankle first contacts the lower level, d) acceleration of the head during the steps, e) EMG activity of the lower limb musculature throughout the step-down procedures. Four experimental conditions will be examined: (1) trials at several step heights under normal sensory conditions, (2) trials without visual cues (subject blindfolded) but with subject verbally informed as to height of the step, (3) trials in which the subject is informed verbally of the step height--sometimes incorrectly informed, (4) trials with a non-predictable step height sequence (subject not informed of step height). Knowledge of the stresses incurred during step-down will offer an important basis for a re-evaluation of the mobility needs of the partially disabled and geriatric populations. More generally, the data will indicate the importance of considering human reflex systems not with the person in a passive state, but rather during performance of active behavior because reflex responses are dependent upon the functional state of the organism.